Wildcard Mask Calculator

Frequently Asked Questions

What is the difference between a wildcard mask and a subnet mask?

In a subnet mask, a 1 bit identifies the network portion. In a wildcard mask, a 0 bit means the ACL must match that bit exactly; a 1 bit is don't-care. They are bitwise inverses serving different purposes.

Can wildcard masks be non-contiguous?

Yes - that is their main advantage over subnet masks. A wildcard like 0.0.0.254 matches every even/odd pair in the last octet. Subnet masks must always be contiguous.

Why does OSPF use wildcard masks in the network statement?

OSPF's network command uses wildcards to specify which local interfaces to include in the routing process. A single wildcard statement can match multiple interfaces across different subnets.

Is a wildcard mask the same as an inverse mask?

Yes - the terms are synonymous. Cisco documentation uses 'wildcard mask'; some other vendors use 'inverse mask.' Both mean the bitwise complement of the subnet mask.

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How It Works

A wildcard mask is the bitwise inverse of a subnet mask: wildcard = 255.255.255.255 - subnet mask, computed per octet (equivalently, flip every bit). Where a subnet mask uses 1 to mean 'this bit must match,' a wildcard uses 0 for must-match and 1 for don't-care. Cisco ACLs and OSPF network statements require wildcards, not subnet masks.

This calculator accepts either a dotted subnet mask (e.g., 255.255.255.0) or a CIDR prefix (e.g., /24 or just 24), inverts it, derives the equivalent CIDR prefix, and - given a base address - shows the exact address range a single ACL line would match and ready-to-paste example ACL and OSPF syntax.

Use Cases

Network engineers use wildcard masks to:

Write Cisco ACL permit/deny statements that match an entire subnet - for example, permit 10.10.1.0 0.0.0.255 matches all 256 addresses in 10.10.1.0/24.

Configure OSPF network statements to advertise specific interface networks into the OSPF process.

Match only odd or even last-octet addresses using non-contiguous wildcards like 0.0.0.254 (last bit don't-care) without writing 128 individual ACL lines.

Match a range of subnets with a single ACL entry using a wildcard that spans multiple /24s or other sized blocks.

Quickly generate the correct wildcard from a given subnet mask when writing firewall policy documentation.

Tips

Never paste a subnet mask where a wildcard is expected - 255.255.255.0 in a Cisco ACL means 'only match addresses that differ in the first octet', which is almost never what you want. Always convert first.

Remember the shorthand: wildcard 0.0.0.0 = match exactly this host (equivalent to the host keyword); wildcard 255.255.255.255 = match any address (equivalent to the any keyword).

Non-contiguous wildcards are powerful but dangerous - 0.0.0.254 matches every even address; verify the range with this calculator before applying to a production router.

ACL order matters - Cisco processes ACL entries top-down and stops at the first match; place more specific entries before broader ones to avoid unintended permit/deny behavior.

The base address should be the network zero-host address - pair the wildcard with the subnet's .0 address (e.g., 10.10.1.0 0.0.0.255), not an arbitrary host address inside the subnet.

FAQ

What is the difference between a wildcard mask and a subnet mask?

In a subnet mask, a 1 bit means 'this bit identifies the network.' In a wildcard mask, a 0 bit means 'this bit must match the source/destination exactly.' They are bitwise inverses of each other and serve entirely different purposes - subnet masks define network boundaries; wildcard masks define which addresses an ACL or routing protocol statement applies to.

Can wildcard masks be non-contiguous?

Yes, and this is their main advantage over subnet masks. A wildcard like 0.0.0.254 (binary 11111110 in the last octet) has the lowest bit as don't-care, matching addresses whose last octet differs only in the lowest bit - effectively matching every even/odd pair. Subnet masks must always be contiguous, so they cannot express this kind of pattern.

Why does OSPF use wildcard masks in the network statement?

OSPF's network command specifies which local interfaces to include in the OSPF process. Using a wildcard allows a single statement to match multiple interfaces on different subnets. For example, network 10.0.0.0 0.255.255.255 area 0 includes all interfaces with addresses in the 10.0.0.0/8 space in area 0.

Is a wildcard mask the same as an inverse mask?

Yes - the terms are synonymous. Cisco documentation uses 'wildcard mask.' Some other vendors and textbooks use 'inverse mask.' Both mean the bitwise complement of the subnet mask.

Wildcard Mask Calculator

Frequently Asked Questions

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